1. Field of the Invention
This invention relates to a top anti-reflective coating material (TARC) and barrier layer and the use thereof in lithography processes. This TARC/barrier layer may be especially useful for immersion lithography in which a liquid such as water is used as the exposure medium between the lens fixture of an exposure tool and the photoresist-coated wafer.
2. Description of the Related Art
Traditionally, top coat materials have been used in photolithography as anti-reflective films on the top of a photoresist. Top anti-reflective coating (TARC) materials can prevent the multiple interference of light that takes place within the photoresist layer during exposure. As a result, the critical dimension (CD) variation of the geometrical features of a photoresist pattern that is caused by the variation in the thickness of the photoresist film can be minimized.
To fully take advantage of the anti-reflective effect of the top coat, the refractive index of the top coat material (nt) should be at about the square root of the multiplication of the refractive index of the exposure medium (nm) and the refractive index of the underlying photoresist (nr). If the exposure medium is air as in the case for “dry” lithography, the optimal refractive index of the top coat material (nt) should be at about the square root of the refractive index of the underlying photoresist (nr), because the refractive index of air is roughly 1. The TARC also requires transparency to the exposure light to prevent intensity loss of the light to the photoresist.
For ease of processing, classic TARC materials are designed to be soluble in both water and aqueous base developer so that they can be applied directly from water solution and subsequently removed by the aqueous base developer during the develop stage. Numerous top coat materials have been developed to meet these two requirements of optimal refractive index and solubility. For example, U.S. Pat. Nos. 5,744,537 and 6,057,080 disclose aqueous-soluble TARC materials comprising a polymeric binder and a fluorocarbon compound, and which have nearly ideal refractive indexes on the order of 1.3-1.4. U.S. Pat. No. 5,879,853 also discloses a TARC material that is removable by a wet process. U.S. Pat. No. 5,595,861 similarly discloses a TARC comprising partially fluorinated compounds, which can also be water soluble. U.S. Pat. No. 6,274,295 discloses a TARC material comprising a light absorbing compound having a wavelength of maximum absorption higher than an exposure wavelength used to expose the photoresist. This TARC can also be water-soluble. U.S. Pat. No. 5,240,812 discloses a protective material for use as an overcoat film for acid catalyzed resist composition to prevent contamination from vapors of organic and inorganic bases. While not specifically disclosed as being a TARC, the overcoat can also be water soluble. Other TARC materials include those disclosed in U.S. Pat. Nos. 5,744,537 and 6,057,080, U.S. Pat. No. 6,503,689, U.S. Patent Application Pub. No. 2003/0211417, U.S. Patent Application Pub. No. 2004/0013971, and U.S. Patent Application Pub. No. 2004/0033436.
Immersion lithography offers the potential to extend the use of optical lithography to print smaller features. In immersion lithography, air is replaced by a liquid medium such as water between the lens and the wafer. Use of a medium with an index of refraction higher than air results in a greater numerical aperature (NA) provided that the projection angle is kept the same in the medium, and therefore allows printing of smaller features.
Immersion lithography, however, has presented some concerns that certain components in the photoresist may leach out to the immersion medium and change the performance of the photoresist, or that the immersion medium may diffuse into the photoresist and affect the acid generation and may then interfere with the chemical amplification mechanism. Much effort has been devoted to modifying photoresist formulations to insure that the above-mentioned problems will not occur during the immersion exposure process, but no photoresist has been developed which is both compatible with water as the proposed immersion medium and has acceptable performance characteristics.
To alleviate these photoresist leaching and solubility problems in immersion lithography, a top coat material can be used between the immersion medium and the resist-coated wafer. Such top coat material can prevent photoresist components from leaching into the immersion medium, and can also prevent permeation of the immersion medium into the photoresist film. One of the requirements for the top coat material, of course, is its insolubility in the immersion medium. Preferably, the top coat material can also act as a TARC layer.
Water has been proposed as the immersion medium for 193 nm immersion lithography. Therefore, classic water-soluble TARC materials such as those described above can not be used as top coats for 193 nm immersion lithography. Moreover, since water has higher refractive index (1.437 at 193 nm) than air (˜1 at 193 nm), the optimal refractive index for TARC materials used for 193 nm immersion lithography is also higher than that of classic TARCs.
Thus, there remains a need for a top coat material that is insoluble in water but soluble in aqueous base developer, and also has desired optical properties so that it can also be used as a TARC.